Photodynamic therapy (PDT) has shown great potential for the treatment of cancer. There is need, however, for the identification of new photosensitizers with improved tumor-localizing capability, improved photodynamic efficacy, and reduced cutaneous side effects. There is also need for deeper understanding of the photochemical, physiological, cellular, and molecular mechanisms of PDT action that will reveal new insights for improvement of the therapeutic effects. This revised program project includes three research projects which individually and together will address these needs. Phthalocyanines have been chosen for these investigations, because of their favorable properties for PDT. Project 1 will prepare new phthalocyanine photosensitizers and determine their photochemical properties. The photosensitizers will be designed for improved tumor localization or bioelimination, employing cationic, amphiphilic, nanoparticle, soft, and polyamine ligands. The new photosensitizers will be evaluated in vitro and in vivo by Core D. Project 2 will test hypotheses as to the mechanisms of photocytotoxicity by examining (a) the activation of signal transduction pathways, (b) combined modality treatments of PDT with ionophores and polyamine inhibitors, (c) the induction of apoptosis by PDT in vitro and in vivo and by conditions thought to be imposed by PDT, and (d) mutagenic effects of PDT. Project 3 will elucidate PDT mechanisms in tumors and normal tissue, focusing on direct vs indirect damage, vascular responses, and immunological/inflammatory responses in RIF-1 tumor-bearing C3H/HeN mice. The contribution of leukocyte sub-populations to tumor regression will be studied, and the roles of cytokines and eicosanoids to the in vivo responses will be determined by in situ hybridization, immunohistology, and cell culture techniques. The projects will be supported by four cores. Core A will maintain and calibrate all of the laser and other light sources for the projects; Core B will provide administrative support; Core C will provide required amounts of established photosensitizers in pure form to the projects; and Core D will carry out standardized evaluations of all new photosensitizers in vitro and in vivo, including measurement of photocytotoxicity, tumor response, pharmacokinetics, and cutaneous photosensitivity. The combined efforts should identify new structures with improved potential for clinical PDT, provide a new dimension of understanding of the critical mechanisms in cancer treatment with PDT, and evaluate the potential for clinically useful modification of PDT responses.